The present invention is generally directed to a system and method for displaying surface information- More particularly, the present invention is directed to a system and method for displaying internal surfaces existing at various depths and locations within a three-dimensional body. The images of the surfaces displayed are typically contained within the interior regions of solid bodies which are examined by computed axial tomographic (CAT) x-ray systems or by nuclear magnetic resonance (NMR) imaging systems either of which is capable of generating three-dimensional arrays of data representative of one or more physical properties at various locations within a three-dimensional object. The images generated in the practice of the present invention are particularly useful in that they provide three-dimensional data for examination by physicians, radiologists and other medical practitioners.
In conventional x-ray systems, a two-dimensional shadow image is created based upon the different x-ray absorption characteristics of bone and soft tissues. A great improvement on the conventional x-ray system as a diagnostic tool is provided by computed axial tomographic systems, which have been developed over the last ten years or so. These so-called CAT systems are x-ray based and initially were used to produce single two-dimensional views depicting transverse slices of a body, object or patient. Three dimensional information was thereafter gleaned from CAT scan data by generating data for a number of contiguous slices and using the inferential abilities of a radiologist to suggest a three-dimensional representation for the various internal organs. In the present invention, shaded and contoured three-dimensional images are generated from the three-dimensional array of data generated by a sequence of such contiguous CAT scans or magnetic resonance imaging scans. The newer magnetic resonance imaging technology possesses the capability to better discriminate between various tissue types, not just between bone and soft tissue and therefore offers the capability for producing more discriminating images in many situations. NMR imaging systems are also capable of generating physiological data rather than just image data. However, whether NMR or CAT systems are employed, data has generally been available only as a sequence of slices, and systems have not generally been available which provide shaded two-dimensional images which accurately depict true three-dimensional views.
Prior work by at least one of the inventors herein has significantly solved some of the major problems associated with the production of high resolution three-dimensional medical images. In particular, a system referred to as "marching cubes" was disclosed in application Ser. No. 741,390 filed June 5, 1985 and now U.S. Pat. No. 4,710,876, issued Dec. 1, 1987. An additional application relating to the display of three-dimensional images and a system referred to as "dividing cubes" was disclosed in application, Ser. No. 770,164 filed on Aug. 28, 1985 and now U.S. Pat. No. 4,719,585, issued Jan. 12, 1988 incorporated herein by reference. At the time of invention, all of the individuals in the present case and these other cases were under an obligation of assignment to the assignee of the present application. Both of these applications are assigned to the same assignee as the present invention. The present invention is in fact applicable to processing either in accordance with the marching cubes system or the dividing cubes system or in accordance with other similar systems.
Attention is now directed to the specific problem solved by the system of the present invention. In the display of three-dimensional images, and more particularly in the display of medical images, one often encounters three-dimensional objects having multiple internal surfaces which occur in layers. For example, three-dimensional data associated with physical measurements of the human head produce data associated with the skin, with the skull, the brain, nasal cavities and various internal soft tissue structures. In a three-dimensional view of the head, for example, there are circumstances in which it would be desirable to be able to effectively strip away skin tissue so as to observe underlying bone tissue. Likewise, there are situations in which it would be desirable to be able to peel away both skin and bone surfaces to reveal underlying structures such as the brain. While the methods of marching cubes and dividing cubes are capable of displaying selected tissues such as all bone or all skin or all brain tissue, it is nonetheless desirable to be able to display selected portions of these structures and/or to simultaneously display them on the same screen so as to more clearly indicate their relationship. This is particularly advantageous as a surgical planning method since it is capable of showing the relationship between various bodily structures. It is noted, however, that while the present invention is particularly directed to the medical imaging arts, there is nothing contained herein which would limit its use thereto. Any three-dimensional measurement process carried out on an object having an internal structure is amenable to processing in accordance with the system of the present invention.
An image of the anatomy typically consists of the visible surfaces of tissues computed by scanning the data and projecting surface patches onto a view plane. In a three-dimensional array of data, the volume element is called a voxel, in an analogy with that of the area element which is referred to as a pixel in two-dimensional situations. In certain other dimensional algorithms, voxel size limited the resolution of three-dimensional reconstructions resulting in images that appear block-like or stepped as compared to having the smooth surfaces of real tissues. Attempts to produce smoother images by averaging over the neighboring voxels however, actually tended to reduce the resolution of the image. Other methods for three-dimensional display generation of images have been based upon measurement of the distance from an imaginary observation point to a patch on the surface of the object and on the estimated surface normal of the patch.
To shade the surface of a three-dimensional image projected onto a view plane, an intensity is calculated from the component of the unit normal vector parallel to the view direction. Surfaces parallel to the view plane are fully illuminated, while those at oblique angles to the view plane are gray and surfaces perpendicular to the view plane are dark or black. The marching cubes and dividing cubes systems estimate the surface normal direction from a gradient vector of the three-dimensional density function. This is a useful estimate since the gradient is perpendicular to surfaces of constant density. Consequently, the gradient vector is parallel to the unit surface normal vector. The unit normal vector is calculated by normalizing the gradient vector at the surface of interest. In the dividing cubes system, the gradient vector defined at each lattice point is linearly interpolated over the voxel to given a local value of the gradient vector at the desired surface. The unit surface normal is the gradient vector divided by its magnitude. Similar variations of the normal direction is also employable in the marching cubes system. The surface that results appears smooth because the interpolated gradient vector continuously varies with the distance across a voxel boundary. This form of gradient shading is preferably employed in both the marching cubes and dividing cubes systems